Abstract: The calculation of the high confidence of low probability of failure (HCLPF) seismic capacity of equipments in nuclear power plants is an important process for seismic probability safety assessment and seismic margin assessment. In this paper, the seismic capacity of the supports of the steam generator was studied. A nonlinear model of the steam generator was built, and the seismic response of the system was calculated iteratively by scaling the ground motion level step by step. The seismic capacity of the supports of steam generator was obtained by this nonlinear time history analysis and was compared with the HCLPE based on the conservative deterministic failure margin method. The results show that the calculating values of two methods are different. The seismic capacity of equipments with nonlinearity should be calculated by nonlinear time history method.